Patent Application
20080152531
The present invention relates to cast irons and more particularly to certain cast irons which have no intentional addition of Mo and which have compositions to provide certain cost and use advantages.
Cast irons are used for exhaust manifold and other automotive component applications. The high-temperature cast irons used for exhaust manifolds, which operate at 650 to 700 degrees C. and above, contain alloying elements such as C, Si, and Mo. The element Mo (molybdenum) is used only up to a maximum of 1% by weight.
The worldwide demand of Mo has sharply increased the price of Mo (approximately 10 fold). Even with only up to 1% by weight of Mo in cast irons for exhaust manifolds, the 10-fold cost increase can add substantial additional cost in connection with continued use of existing cast irons.
The increased cost of Mo has posed a need for cast irons that are substantially free of Mo; i.e., for cast irons that do not include an intentional addition of Mo as an alloying element.
The present invention provides certain cast irons which have no intentional addition of Mo and which have compositions to provide certain cost and use advantages over conventional Mo-bearing cast irons. These advantages include, but are not limited to, lower cost and a higher ferrite-to-austenite transformation temperature resulting in higher use temperatures than conventional Mo-bearing cast irons.
In practice of an embodiment of the invention, a cast iron is provided wherein Si is present in an amount of about 3.5 weight % to about 6.0 weight % and wherein Mo is absent or present as an impurity element.
In an illustrative embodiment of the invention, a cast iron is provided consisting essentially of, in weight %, about 2.0% to about 4.0% C, about 3.5% to about 6.0% Si, and balance Fe wherein Mo is absent, such as substantially 0 weight %, or present as an impurity element, such as not exceeding about 0.01 weight %. Si preferably is about 4.5% to about 6.0% by weight and even more preferably about 4.75% to about 5.5%. C preferably is about 2.5% to about 3.5% of the cast iron composition.
In a further illustrative embodiment of the invention, a nodular cast iron is provided having a composition, in weight %, consisting essentially of about 2.0% to about 4.0% C, about 3.5% to about 6.0% Si, up to about 2.0% Cr, up to about 2.0% Mn, up to about 0.75% Ni, up to about 2.5% W, up to about 2.5% Cu, up to about 1.0% V up to about 1.0% Ti, an effective amount of a nodularizing agent, and balance Fe wherein Mo is absent or present as an impurity element.
One or more of Cu, W, V, or Ti can be present in the cast iron composition in a respective amount of about 0.25% to about 2.5% for Cu, about 0.25% to about 2.5% for W, about 0.025% to about 0.2% for V, or about 0.1% to about 0.25% for Ti.
Cast irons pursuant to the invention that are free of Mo or have only impurity Mo are advantageous to provide a higher ferrite-to-austenite transformation temperature (e.g. AC1 temperature) that is about 50 degrees C. higher than a conventional Mo-containing cast iron, although the microstructure is similar in its graphite content to that of Mo-containing cast iron. For example, cast irons of the invention can exhibit an AC1 temperature of about 880 degrees C. and above. Cast irons of the invention can operate at temperatures of 650-700 degrees C. and above, such as 750 degrees C., encountered in exhaust manifold applications.
Moreover, the cast irons of the invention are advantageous to provide a higher tensile strength at room temperature and above (e.g. at 600 degrees C.) than a conventional Mo-containing cast iron. For example, the cast irons of the invention can have an ultimate tensile strength above 90 ksi at 23 degrees C. (room temperature-RT) and an ultimate tensile strength above 30 ksi at 600 degrees C. These advantages are achieved at lower alloy cost since no intentional Mo addition is made to the cast iron.
These and other advantages of the present invention will become more readily apparent form the following detailed description taken with the following drawings.
a and 9b are photomicrographs of the cast microstructures of evaluated cast irons GMO-10 and GMO-10A, respectively, with a 2% Nital etch.
a and 10b are photomicrographs of the cast microstructures of comparison cast irons GM-1 and GM-2, respectively, with a 2% Nital etch.
The present invention provides cast irons and, in particular, nodular cast irons, which have no intentional addition of Mo and which have alloy compositions to provide certain cost and use advantages over conventional Mo-bearing cast irons, in particular to increase the AC1 temperature (onset or start of the ferrite-to-austenite transformation). Cast irons of the invention have alloy compositions wherein Si is present in a higher amount of about 3.5 weight % to about 6.0 weight % and wherein Mo is absent (i.e. substantially 0 weight % Mo) or present as an impurity element such as not exceeding about 0.01 weight % of the cast iron composition. Si preferably is about 4.5% to about 6.0% by weight and even more preferably about 4.75% to about 5.5% by weight of the cast iron composition. Mo preferably is absent from the cast iron composition.
A particular illustrative cast iron is provided consisting essentially of, in weight %, about 2.0% to about 4.0% C, about 3.5% to about 6.0% Si, and balance Fe wherein Mo is absent or present as an impurity element of the cast iron composition. C preferably is about 2.5% to about 3.5%
Another illustrative embodiment of the invention provides a nodular cast iron having a composition, in weight %, consisting essentially of about 2.0% to about 4.0% C, about 3.5% to about 6.0% Si, up to about 2.0% Cr, nodularizing agent such as Mg and/or Ce, up to about 2.0% Mn, up to about 0.75% Ni, up to about 2.5% W, up to about 2.5% Cu, up to about 1.0% V up to about 1.0% Ti, an effective amount of a nodularizing agent such as about 0.01% to about 0.10% of Mg and/or Ce, and balance Fe wherein Mo is absent or present as an impurity element. A nodular or ductile cast iron is provided when Mg, Ce or other nodularizing agent is present in the composition in an effective amount to nodularize the graphite in the as-cast microstructure.
One or more of Cu, W, V, or Ti can be present in a respective amount of about 0.25% to about 2.5% for Cu, about 0.25% to about 2.5% for W, about 0.025% to about 0.2% for V, or about 0.1% to about 0.25% for Ti of the cast iron composition.
A certain preferred Mo-free cast iron nominally consists essentially of, in weight %, about 3.0% C, about 5.0% Si, about 0.03% Cr, about 0.5% Mn, about 0.03% Ni, about 0.3 to about 0.5% W, an effective amount of Mg for nodularization, and balance Fe.
Another preferred Mo-free cast iron nominally consists essentially of, in weight %, about 3.0% C, about 5.0% Si, about 0.03% Cr, about 0.5% Mn, about 0.03% Ni, about 0.3 to about 0.5% W, about 0.4 to about 0.5% Cu, an effective amount of Mg for nodularization, and balance Fe.
Still another preferred Mo-free cast iron nominally consists essentially of, in weight %, about 3.0% C, about 5.0% Si, about 0.03% Cr, about 0.5% Mn, about 0.03% Ni, about 1.0 to about 1.5% W, an effective amount of Mg for nodularization, and balance Fe.
A still further preferred Mo-free cast iron nominally consists essentially of, in weight %, about 3.0% C, about 5.0% Si, about 0.03% Cr, about 0.5% Mn, about 0.03% % Ni, about 0.3 to about 0.5% W, about 0.4 to about 0.5% Cu, about 0.1% of each of V and/or Ti, an effective amount of Mg for nodularization, and balance Fe.
Cast irons pursuant to the invention that are free of Mo or have only impurity Mo are advantageous to provide an AC1 temperature that is about 50 degrees C. higher than a conventional Mo-containing cast iron, although the as-cast microstructure preferably is similar in its content of graphite to that of Mo-containing cast irons. A higher AC1 translates to a higher use or operating temperature of the cast iron. For example, cast irons of the invention can exhibit an AC1 temperature of about 880 degrees C. and above. Cast irons of the invention can operate at temperatures of 650-700 degrees C. and above, such as 750 degrees C., encountered in exhaust manifold applications.
As mentioned above, the as-cast microstructure of cast irons pursuant to the invention preferably is similar in its content of graphite to that of Mo-containing cast irons. For example, cast irons pursuant to the invention can have about 2.75 to about 3.5 weight % graphite content, such as nominally about 3 weight % graphite content, as described and determined below.
Moreover, cast irons of the invention are advantageous to provide a higher tensile strength (e.g. about 10 ksi higher) at room temperature and 600 degrees C. than a conventional Mo-containing cast iron. For example, the cast irons of the invention can have an ultimate tensile strength above 90 ksi at 23 degrees C. (room temperature) and an ultimate tensile strength above 30 ksi at 600 degrees C. These advantages are achieved at lower alloy cost since no intentional Mo addition is made to the cast iron.
Certain low-Mo or Mo-free cast iron alloys which were evaluated in arriving at the present invention have compositions shown in Table 2 where values are weight %. These cast iron alloys designated GMO alloys were compared to Mo-containing cast iron alloys designated GM-1 and GM-2 whose compositions are shown in Table 1 where values are weight %. The evaluation and comparison initially proceeded using computational modeling to study the microstructural features and transformation temperatures for the comparison Mo-containing cast iron alloys GM-1 and GM-2 and was followed by validation of model predictions with measured transformation temperatures, microstructural phases (e.g. graphite nodule content and other phases) and mechanical properties at RT (room temperature), 600 degrees C., and 800 degrees C. from alloy casting trials. The computational modeling to study the microstructural features and transformation temperatures was effected using commercially available modeling software known as Thermocalc™ software. The software has the data base for iron base alloys and is able to predict stable phases of the alloys under equilibrium conditions as a function of temperature. The phase prediction permits knowing the transformation temperatures in the solid state, such as ferrite to austenite, and the solid to liquid state, such as melting.
Comparison Mo-containing cast irons GM-1 and GM-2 are currently used in making exhaust manifolds for internal combustion engines. Computational modeling was used to compute the phase stability as a function of temperature and phase content for Mo-containing alloys in
A goal of the computational modeling was to create the GMO cast iron compositions that would yield the same or similar graphite content and very similar other phase content (phases other than graphite) as in comparison cast irons GM-1 and GM-2. The calculated values of some of the new evaluated cast iron compositions are included in Table 3 where values are in weight % It can be noted from Table 3 that the compositions of GMO-1, -2, -3, and -4 alloys can produce the same graphite content as comparison cast irons GM-1 and GM-2. The GMO-2 alloy had 0.2 weight % Mo to predict its effect when W is added. This would give an idea of Mo impurity effects if Mo ever got as high as 0.2% Mo. As is shown later, this alloy GMO-2 was not deemed within the invention as a result of its lower AC1 transformation temperature than the comparison cast irons GM-1 and GM-2.
The GMO-1 and GMO-2 alloys have similar metal carbide M6C phases as present in comparison cast irons GM-1 and GM-2, and the total phase content for GMO-1 and GMO-2 is even higher than GM-1 and GM-2 at the temperatures evaluated. GMO-1 included metal carbide phase MC-SHP, which is mostly WC, Table 3 and
Phase calculations for selected GMO alloys (from Table 2) with desired phase contents are shown in Table 4 where values are in weight %, and from this table, the following observations are worth noting: 1. The evaluated GMO compositions of Table 4 have about 3 wt % graphite. 2. GMO-10A has similar range of M6C as comparison cast iron GM-2 but also contains other phases. The total phase content for GMO-10A is higher than GM-2 and thus can yield improved properties. 3. GMO-18 and GMO-19 are very similar and have M6C phase similar to comparison cast iron GM-2 but also contain the M(C,N) and Cu phase. 4. GMO-21 and GMO-22 are the leaner alloy compositions (see Table 2) with M6C (similar to comparison cast iron GM-2) but total phase content made up with the presence of M(C,N) and Cu contents.
aTotal phase content other than graphite.
aExcluding graphite content.
Another goal in the development and evaluation of the GMO cast iron alloys is to increase upper use temperature in service. The maximum upper temperature of cast irons is guided by the transformation temperature (AC1) where ferrite begins to transform to austenite (see
aCalculation not performed.
Alloy GMO-10 was the first alloy subjected to the actual melt trial under foundry conditions. The compositions chosen for alloy GMO-10 are shown in Table 6. A 200-lb heat of AIM composition of GMO-10 was air-induction melted and cast into sand molds and air cooled. Half of the heat (about 100 lb) was poured into test bars and the remainder was adjusted to add 1 wt % W. This W-containing composition was called alloy GMO-10A. The charge materials that were melted comprised commercially available, powder and/or solid elemental constituents. The actual compositions achieved for alloys GMO-10 and GMO-10A are shown in Table 6 where values are weight %. This table shows that the AIM composition was met for most of the alloying elements, with exception of W, which was on the high side of the AIM value of 1%. Impurity Mo was less than 0.01 weight %. The cast bars of GMO-10 and GMO-10A had dimensions of 6½ inches length, 1⅜ inches width, and 1 inch thickness.
Differential scanning microscopy was used to determine the transformation temperature for the comparison cast iron GM-2 and the evaluated alloys GMO-10 and GMO-10A (see
Based on observations for GMO-10A, this evaluated cast iron could be considered as an alternate Mo-free cast iron to comparison cast iron GM-2.
aAlloy shows a 50° higher transformation temperature.
The optical microstructure of cast bars of GMO-10 and GMO-10A (
Tensile data for the comparison cast irons GM-1 and GM-2 and the evaluated cast iron alloys GMO-10 and GMO-10A are shown in
The casting second trial was carried out on alloys GMO-21 and GMO-22. For each of these alloys, two variants were targeted (GMO-21A/GMO-21B and GMO-22A/GMO-22B). Similar to GMO-10 trial, each of the compositions was air-induction melted into 200-lb heats and split into halves, each for targeting A and B compositions. The compositions were cast into sand molds and air cooled. The charge materials that were melted comprised commercially available, powder and/or solid elemental constituents. Table 9 shows target and actual compositions in weight % achieved for alloys GMO-21A, -21B, -22A, and -22B. This table shows that for alloys GMO-21A and GMO-21B and GMO-22A, the target and actual analyses were very close. However, GMO-22B did not meet its target values of V and Ti.
The transformation temperature for GMO-21A, -21B, -22A, and -22B was measured using the DSC,
aPredicted using actual chemistry.
bMeasured transformation values are ~50° C. higher for new GMO alloys as compared to the comparison GM-1 and GM-2 alloys.
cPredicted using nominal chemistry.
dAn exception; needs to be confirmed.
Tensile data for Trial #1 and Trial #2 alloys are presented in Table 11. Tensile properties, 0.2% yield strength, and ultimate tensile strength for the GMO alloys from Trial #1 and Trial #2 are compared with the comparison Mo-containing cast iron GM-2 in
Total elongation of various GMO alloys at RT is compared with comparison cast iron GM-2 in
Measured values of the transformation temperature for comparison cast iron GM-2 and the evaluated GMO alloys are plotted as a function of Si content in
Yield and ultimate tensile strength data on comparison Mo-containing cast irons GM-1 and GM-2 and Mo-free alloys (GMO-10, -10A, -21A, -21B, -22A, and -22B) are plotted as a function of total phase content (other than graphite) in
Although certain detailed embodiments of the invention are disclosed herein, those skilled in the art will appreciate that the invention is not limited to these embodiments but only as set forth in the appended claims.
This invention was made with Government support under Agreement No. ERD-96-XJ190 awarded by the Department of Energy. The Government may have certain rights in the invention.
